1. Field of the Invention
This invention relates to a photographic support and more particularly, it is concerned with an improved photographic support in which deposition or sedimentary adhesion of volatile diffusible substances or exudative substances, which are contained in polyester employed for making an inner layer of the support, onto the surfaces of the support is prevented from occurring by covering both sides of the inner layer with a different type of polyester.
2. Description of the Prior Art
Polyester films have often been used as photographic supports in view of their intrinsically desirable properties, such as high physical strength, durability, dimensional stability with respect to variations in temperature and humidity, and transparency.
However, the addition of commonly used additives, such as an antioxidant, an ultraviolet ray-absorbing agent, a thermal stabilizer, flame retardant agent, etc., to a polyester film support, which has been tried with the intention of further improvement upon such properties of polyester as described above, and the presence of remaining oligomers in the polyester film in considerable amounts has frequently caused troubles that the additives or the oligomers "smoke" or accumulate at the film surface due to their volatile diffusibility or exudative property in a film-forming process. For example, such problems can occur: (1) when passing the polyester through a hopper of an extruder (in an extreme case, even when they are compatible with the polyester to some degree); (2) when passing through the vicinity of a die forming part of an extruder; (3) when producing a fresh film just after the polyester has exited from the die; or (4) in a subsequent process of drawing the film produced or treating the drawn film with heat. High temperatures are generally used in the above-described film-forming and film-treating processes because polyesters have, in general, heat resistant properties, e.g., a high melting point, and it is also excellent in other thermal characteristics. In addition, even though problems may not occur during the film-forming process, unfavorable phenomena similar to those described above may take place during processing, for example, in a process wherein the photographic support is exposed to a high temperature atmosphere for a long period of time.
On the other hand, as disclosed in Japanese Patent Application (OPI) Nos. 101417/78 and 146773/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), copolymerized polyethylene terephthalate film containing as a copolymerizing component polyalkylene glycol in a fraction of 3 to 20% by weight has lower glass transition point than usual polyester film prepared by using, in general, ethylene glycol as a copolymerizing component and, therefore, for example, when the above-described copolymerized polyethylene terephthalate film is allowed to stand at a high temperature of 100.degree. C. or above, oligomers exude from the interior of the film toward the surface thereof, accumulating at the surface of the film, resulting in a change in the appearance of the surface, viz., from transparent to milky. That is to say, there is an increase in haze and a large decrease in transparency.
Accordingly, although the water permeability of this polyester is high due to the use of polyalkylene glycol as a copolymerizing component thereof, and therefore this polyester is particularly useful as a film support for instant photography, the aesthetic value of the image is markedly decreased when imagewise exposure is performed or when the image formed is observed from the side of such a polyester film support in which an increase in haze has occurred (such as by allowing the photographic film to stand in a high temperature atmosphere when a subbing layer and various photographic layers are provided on such a film support, or when such layers processed).
Further, since volatile diffusible or exudative substances, such as the above-described various kinds of additives, migrate and form deposits on the surface of a polyester film support in the support film-forming process or in the subsequent processes of providing an undercoating for various photographic layers, coating various photographic layers and/or processing the layers so provided, the high transparency necessary for the photographic film support cannot be attained. Also, undesirable delamination of a photographic layer through the lowering in adhesiveness of the film support to the photographic layer coated thereon can occur. Furthermore, the deposits accumulated on the film support can exert chemically undesirable effects upon the photographic layers, such as causing photographic fog, desensitization, and spots.